Alternative video sync detector

ABSTRACT

A circuit for generating video synchronization timing signals includes a negative peak detector (FIG.  5 ) for following variations of a composite video signal (FIG.  1 ), rather than clamping the most negative voltage of the composite video signal. The negative peak detector provides a voltage level V TIP  representative of the voltage at the synchronization tip of the composite video signal. A sample and hold circuit ( 700, 702, 704 ) is used to add an offset V SLICE  to V TIP , V SLICE  being a voltage level of the breezeway, color burst, or back porch segments of the composite video signal, or a combination of these segments. The sample and hold circuit generates a signal V REF , and is connected by a resistor divider ( 708,710 ) to the negative peak detector to form the signal V TIP +V SLICE  provided to an amplifier ( 606 ) functioning as a comparator. The signal V SLICE +V TIP  is compared in comparator ( 606 ) with the composite video signal to provide an overall circuit output. Buffering is provided at the input of the negative peak detector by amplifier ( 600 ) to reduce any DC offset from the diode of the negative peak detector. To prevent amplifier DC offset error voltages from affecting the perceived V SLICE  level, an amplifier ( 800 ) can be connected in a first position T TIP  as part of a negative peak detector to store V TIP  on a capacitor, in a second position T H  as part of a sample and hold circuit to store V REF  on a capacitor, and in a third position T COMP  to compare V SLICE +V TIP  measured from the capacitors with the composite video signal to generate the overall circuit output.

PRIORITY CLAIM

This application is a continuation of and claims priority to U.S. patentapplication No. 09/398,375, filed Sep. 17, 1999 (now U.S. Pat. No.6,573,943).

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to the field of video circuits. Moreparticularly, this invention relates to a circuit for detectingsynchronizing pulses embedded in composite waveforms of a video signal.

2. Description of the Related Art

FIG. 1 illustrates components of a composite video waveform. Thecomposite waveform contains: a horizontal sync pulse or sync tip usedfor receiver scan timing; a “breezeway” where the level is a referencefor video intensity; a color burst which is a series of sinewaves at avery precise frequency and phase, used as a color reference; a backporch which is a level reference similar to the breezeway occurringafter the colorburst segment; and the picture occurring after the backporch, the picture being any possible signal up to a maximum level, andwhose content is unpredictable to receiver electronics.

The video receiver systems must discover timing details from the synctip. Unfortunately, the sync tip almost never has a known DC level. Infact, most composite signals are AC coupled and the average DC levelvaries unpredictably with picture content.

One method for providing a video signal timing reference is to use acircuit which uses the most negative going feature of the compositesignal as a reference level. The composite video signal standard whichis predominantly used in North America, the National Television SystemsCommittee (NTSC) standard, was designed to enable such a reference levelto be set approximately 50 years ago.

A prior art circuit for setting a reference level at the most negativefeature of a composite waveform is the clamping circuit shown in FIG. 2.The circuit includes a capacitor 200 having an input receiving thecomposite video signal input, and an output providing the compositevideo signal with its most negative voltage clamped to 0 volts. Thecircuit further includes a diode 202 and current sink 204 connecting theoutput of the capacitor 200 to ground. The diode 202 is assumed to beideal so that it generates no DC offset. The current sink 204 provides asmall pull down current I_(PULLDOWN) to discharge the capacitor 200 andallow the clamped output signal to follow the varying content of thecomposite input.

A clamped output signal from the circuit of FIG. 2 is shown in FIG. 3.As shown in FIG. 3, the diode of FIG. 2 forces the capacitor coupledcomposite video signal's most negative voltage, here the sync tipvoltage (V_(TIP)), to ground level. Because the composite video signalprovides transient currents, clamping may distort the composite signaland may be an undesirable method.

To provide a synchronization (sync) timing signal, the clamped output ofthe circuit of FIG. 2 is provided to a first terminal of comparator 400shown in FIG. 4, while a DC offset voltage is provided to the secondterminal of comparator 400. The sync timing signal is generated when thecomparator output transitions. A DC voltage offset generator 402provides the DC offset voltage at a desired “slice level” (V_(SLICE)),as shown in FIG. 3, so that the sync timing signal is generated on anedge of the sync pulse at the voltage V_(SLICE) approximately midwaybetween the sync tip voltage level V_(TIP) and the breezeway voltagelevel.

SUMMARY OF THE INVENTION

The present invention provides a circuit for following variations of thecomposite video signal, rather than clamping the most negative voltageof the composite video signal.

The present invention includes a negative peak detector with an inputreceiving the composite video signal and an output coupled to a firstinput of a first amplifier the first amplifier functioning as acomparator. The second input of the comparator receives the compositesignal, and the output of the comparator provides a synchronizationtiming signal.

In one embodiment, the present invention further provides buffering atthe input and output of the negative peak detector. Buffering isprovided to the input with a second amplifier having a noninvertinginput receiving the composite video signal. The inverting input of thesecond amplifier is connected to a first terminal of a diode of the peakdetector and also to a current source in the negative peak detector. Theoutput of the second amplifier is connected to the second end of thediode of the negative peak detector. The second amplifier serves tobuffer the composite video signal from the current source. Buffering atthe output of the negative peak detector is provided by a thirdamplifier connected in a voltage follower configuration between theoutput of the negative peak detector and the comparator.

In one embodiment, the present invention also includes a voltage slicelevel offset generator connecting the output of the negative peakdetector to the comparator. The slice level offset generator includes asample and hold circuit and a resistor divider. The sample and holdcircuit is configured to sample the composite video signal during thebreezeway segment, color burst segment, or back porch segments of thecomposite video signal, or any combination of the segments. The outputof the sample and hold circuit then provides a sample of these segmentsV_(REF) to a first end terminal of the resistor divider. The second endterminal of the resistor divider is driven by the buffered output of thenegative peak detector which provides a synchronization tip voltagesignal V_(TIP), and the center terminal of the resistor divider isprovided to the first input of the comparator. The comparator output canthen provide a timing signal transitioning at a point V_(SLICE) on thecomposite signal halfway between V_(TIP) and V_(REF).

In another embodiment in accordance with the present invention,circuitry is configured to reduce amplifier DC offset which can causeerrors in a perceived V_(SLICE) level. The circuitry includes a firstamplifier which receives the composite video signal and is connectableby switches in one of three positions T_(COMP), T_(TIP) and T_(H). Inthe T_(COMP) position the first amplifier acts as a comparator with nofeedback to compare the value V_(SLICE)+V_(TIP) with the composite videosignal. V_(SLICE) is set between V_(TIP) and V_(REF) based on valuesstored on capacitors in the circuit. The T_(COMP) position is used priorto the negative going synchronization tip edge of the composite videosignal. After the negative going edge of the synchronization tip, thecircuit is set in the T_(TIP) position. In the T_(TIP) position, theoutput of the first amplifier is disconnected from providing thesynchronization timing output, and is connected to provide buffering fora negative peak detector to store T_(TIP) on a capacitor. After thesynchronization tip, during the breezeway, colorburst or back porchsegments of the composite video signal, or during a desired combinationof these segments the circuit is connected in the T_(H) position. In theT_(H) position, the first amplifier forms part of a sample and holdcircuit for storing a value V_(REF) on a capacitor. After the desiredperiod for T_(H), the circuit is again connected in the T_(COMP)position for detection of the next negative going synchronization tipedge.

BRIEF DESCRIPTION OF THE DRAWINGS

Further details of the present invention are explained with the help ofthe attached drawings in which:

FIG. 1 shows a composite video signal;

FIG. 2 shows a prior art circuit for clamping the most negative voltageof a composite video signal;

FIG. 3 shows a clamped composite video signal output from the circuit ofFIG. 2;

FIG. 4 shows circuitry used with the circuitry of FIG. 2 to generate asynchronization timing signal;

FIG. 5 shows a negative peak detector according to the present inventionfor providing a voltage reference at the sync tip voltage level of acomposite video signal;

FIG. 6 shows the negative peak detector of FIG. 5 with buffering at itsinput and output, and with additional circuitry to generate a synctiming signal;

FIG. 7 shows the video synchronization signal generating circuit of FIG.6 with components for a slice level offset generator providing anadaptive V_(SLICE) value; and

FIG. 8 shows a video synchronization signal generating circuit enablingelimination of amplifier DC offset voltages.

DETAILED DESCRIPTION

The present invention provides a circuit for establishing a sync tipbaseline without clamping the most negative portion of the compositesignal to a known value. Instead of clamping the sync tip, the negativepeak detector follows variations in the composite signal. The circuit ofthe present invention includes a negative peak detector as shown in FIG.5.

As shown in FIG. 5, the negative peak detector is a rectifier includinga p-n type bipolar diode 500, or other rectifying element, with thecomposite video signal provided to the n terminal and the sync tip leveloutput provided at the p terminal. The negative peak detector alsoincludes a weak current source 502 and a capacitor 504 connecting the pterminal of diode 500 to ground. The current source 502 functions tocharge the capacitor 504. The output of the negative peak detector is areference voltage which tracks the sync tip voltage level V_(TIP).

FIG. 6 shows the negative peak detector of FIG. 5 with buffering at itsinput and output, and with additional circuitry to generate a synctiming signal. Components carried over from FIG. 5 to FIG. 6, as well ascomponents carried over in subsequent drawings, are similarly labeled.

Buffering at the input of the negative peak detector is provided by anoperational amplifier 600. Amplifier 600 has a noninverting inputreceiving the composite video signal, an inverting input connected tothe p terminal of the diode 500, and an output connected to the nterminal of diode 500. The amplifier 600 forms an operational rectifierwith the diode 500 and, thus, buffers current drawn from the currentsource 502 from the composite video signal. The amplifier 600 alsoeliminates diode offset voltage errors with diode 500 not being ideal.

Buffering at the output of the negative peak detector is provided byoperational amplifier 602. The amplifier 602 is connected as a voltagefollower with a noninverting input connected to the p terminal of thediode 500, and its output and inverting input connected together. Theamplifier 602 buffers the sync tip level voltage V_(TIP) on thecapacitor 504 from any load.

An amplifier 606 configured as a comparator, and a slice level generator604 are included with the amplifiers 600 and 602 and negative peakdetector to provide a synchronous timing signal. The composite videosignal is provided to the noninverting input of the comparator 606. Theoutput of amplifier 602 is provided through a slice level offsetgenerator 604 to the inverting input of the comparator 606.Synchronization timing signals are produced at the output of comparator606.

In one embodiment, the slice level offset generator 604 generates afixed DC voltage V_(SLICE) which is added to V_(TIP) at the output ofamplifier 602. The output of the comparator 606 will then transitionwhen the composite video signal transitions through the voltage levelV_(TIP)+V_(SLICE).

Because the composite video signal has a varying amplitude, it may bedesirable to have an adaptive rather than a constant V_(SLICE) value.FIG. 7 shows specific components for an embodiment of the slice levelgenerator 604 which can provide such an adaptive V_(SLICE) value.

The offset slice generator 604 of FIG. 7 and includes a sample and holdcircuit made up of amplifier 700, switch 702 and capacitor 704. Thenoninverting input of amplifier 700 receives the composite video signal,while the inverting input of the amplifier is connected by the switch702 to its output. The noninverting input of amplifier 700 is alsoconnected to capacitor 704. The switch is selectively switched by asignal T_(H) which is timed to close the switch during the breezeway,color burst or back porch segments of the composite video signal, orduring any combination of the breezeway, color burst or back porchsegments. The capacitor 704, thus, stores a voltage V_(REF) equal to thesampled voltage of the breezeway, color burst, or back porch segments,or the desired combination of these segments.

The voltage held by the capacitor 704 is buffered by operationalamplifier 706 and applied to a resistor divider made up of resistors 708and 710. The operational amplifier 706 is connected in a voltagefollower configuration with its noninverting input connected to theinverting input of the amplifier 700, and its inverting input connectedto its output. The output of the amplifier 706 is connected to a firstterminal of resistor 708, while the output of amplifier 602 is connectedto the first terminal of resistor 710. The second end of resistors 708and 710 are both connected to the noninverting input of comparator 606.

With resistors 708 and 710 having equal values, the output of thecomparator 606 will transition when the composite video signal passesthrough V_(TIP)+V_(SLICE) with V_(SLICE) being halfway between V_(TIP)and V_(REF). The present invention can, thus, provides an adaptiveV_(SLICE) value.

With the circuit of FIG. 7, the offset of all four amplifiers 600, 602,700 and 706, and the comparator 606 can add to cause undesirable errorsin the desired signal V_(TIP)+V_(SLICE). Errors in the value forV_(TIP)+V_(SLICE) show as timing errors since the input to the circuitof FIG. 7 does not have a large slew rate, and even non-functionality ifthe errors accumulate large compared to undersized input signals.

FIG. 8 shows an embodiment of a video synchronization signal generatingcircuit in accordance with the present invention which enableselimination of amplifier DC offset voltages.

The circuit of FIG. 8 includes an amplifier 800 and switches 802 and 804to selectively connect the amplifier 800 to different components. Thenoninverting (+) input of the amplifier 800 receives the composite videosignal input. The switch 802 connects the output of the amplifier 800 toone of three terminals labeled T_(H), T_(COMP) and T_(TIP). The switch804 connects the inverting (−) input of the amplifier 800 to one ofthree terminals labeled T_(H), T_(COMP) and T_(TIP) to correspond withthe connections for switch 802. The switches 802 and 804 are controlledto switch together to a terminal with a common label.

When the switches 802 and 804 are set to the T_(COMP) connections, theamplifier 800 acts as a comparator with no feedback to compare the valueV_(SLICE)+V_(TIP) with the composite video signal. The switches 802 and804 are preferably set to the T_(COMP) positions during a majority ofthe composite video signal prior to the negative synchronization tipedge.

With the switches 802 and 804 in the T_(COMP) position, the (−) input ofthe amplifier 800 is connected to the output of amplifier 602. Like theamplifier 602 in FIGS. 6 and 7, the amplifier 602 in FIG. 8 is connectedin a voltage follower configuration with its (+) input connected to anegative peak detector formed by diode 500, capacitor 504 and currentsource 502. The diode 500 is disconnected from the output of theamplifier 800 when the switches 802 and 804 are connected in theT_(COMP) position, but the capacitor 504 will store the voltage valueV_(TIP) of the synchronization tip. The value V_(TIP) is measured andstored by capacitor 504 after the negative edge of the synchronizationtip signal with the switches 802 and 804 connected to the T_(TIP)connections, as discussed in detail to follow.

The output of the amplifier 602 is connected to the (−) input of theamplifier 800 through a resistor 710 when the switches 802 and 804 arein the T_(COMP) position. Also, the (−) input of the amplifier 800 isconnected to the output of amplifier 706 through resistor 708. Theamplifier 706 is connected in a voltage follower configuration similarto the amplifier 706 of FIG. 7. The capacitor 704 is charged up to avoltage V_(REF) equal to the sampled voltage of the breezeway, colorburst, or back porch segments, or the desired combination of thesesegments. The capacitor 704 is charged up to V_(REF) when the switches802 and 804 are in the T_(H) position as described in more detail tofollow.

With resistors 708 and 710 having equal values, the output of theamplifier 800 will transition when the composite video signal passesthrough V_(TIP)+V_(SLICE), V_(SLICE) being half way between V_(TIP) andV_(REF). Further, any offset in amplifier 800 is stored in capacitors504 and 704 and is superimposed on the V_(SLICE) value by amplifiers 602and 706 and fed back to the (−) input of amplifier 800. Functioning as acomparator, the offset of the amplifier 800 is now cancelled withrespect to the input. Any offset of amplifiers 602 and 706 will bereduced by the voltage gain of amplifier 800 with respect to the inputof the synchronous detector circuit.

After the output of the synchronous detector circuit signals thenegative edge of the synchronization tip, the switches 802 and 804 areset to T_(TIP). The amplifier 800 then is disconnected from providingthe sync timing output signal and acts only as a buffer for a negativepeak detector. The amplifier 800 has a (+) input receiving the compositevideo signal as in FIGS. 6 and 7, but instead of having the (−) inputconnected directly to the p terminal of the diode 500, the (−) input isbuffered from the diode 500 through the voltage follower amplifier 602.As connected, the amplifier 800 will reduce the offset of diode 500 aswell as the offset of amplifier 602.

The switches 802 and 804 are left in the T_(TIP) position until thecapacitor 504 charges up to the synchronization tip voltage levelV_(TIP). After a time period for the capacitor 504 to adequately charge,the switches 802 and 804 are set to T_(COMP) again, waiting for thepositive going edge of the synchronization tip.

After the positive-going edge of the synchronous tip is perceived, theswitches 802 and 804 are set to the T_(H) connections. As with thecircuit of FIG. 7, the time period for T_(H) can be during thebreezeway, color burst or back porch segments of the composite videosignal, or during any combination of the breezeway, color burst or backporch segments. The switches 802 and 804 are connected to the T_(H)connections during the desired segments and returned to the T_(COMP)connections afterward.

With the switches 802 and 804 connected in the T_(H) position, theamplifier 800 functions similar to the amplifier 700 of FIG. 7 withcapacitor 704 to form a sample and hold circuit. Rather than beingconnected directly to the capacitor 604 like the (−) input of amplifier700 of FIG. 7, the (−) input of the amplifier 800 is connected to thecapacitor 704 through the voltage follower amplifier 706. The offset ofamplifier 706 is reduced by this loop.

With the switches 802 and 804 connected in the T_(H) position, thecapacitor 704 will charge up to and store a voltage V_(REF) equal to thesampled voltage of the breezeway, color burst, or back porch segments,or the desired combination of these segments. After the desiredsegments, the switches 802 and 804 will be placed in the T_(COMP)positions until the negative going edge of the synchronization tip isdetected again.

Although the invention has been described above with particularity, thiswas merely to teach one of ordinary skill in the art how to make and usethe invention. Many modifications will fall within the scope of theinvention, as that scope is defined by the claims which follow.

1. A method for detecting synchronization pulses embedded in a compositevideo signal, comprising: receiving the composite video signal;detecting a synchronization tip voltage level V_(TIP) of the compositevideo signal; producing a voltage level V_(SLICE); summing the voltageV_(TIP) and the voltage V_(SLICE); and comparing the composite videosignal to the sum of V_(TIP)+V_(SLICE) to thereby produce asynchronization timing output.
 2. The method of claim 1, whereinV_(SLICE) comprises a substantially constant voltage level.
 3. Themethod of claim 1, wherein V_(SLICE) comprises an adaptive voltagelevel.
 4. The method of claim 3, wherein producing the adaptive voltagelevel V_(SLICE) comprises: producing a reference voltage level V_(REF)based on samples of at least one of a breezeway segment, a color burstsegment and a back porch segment of the composite video signal; andproducing the adaptive voltage level V_(SLICE) based on the voltageV_(TIP) and the voltage V_(REF).
 5. The method of claim 4, wherein theadaptive voltage level V_(SLICE) is produced such that it issubstantially halfway between the voltage V_(TIP) and the voltageV_(REF).